An SbW (safe-by-wire) is proposed as one of communication protocols used for wiring data in a vehicle. In the SbW, the power is supplied and the communication is effected by using two wires only for use in those applications where it is desired to lower the probability of breakage in the wiring.
As shown in FIG. 14, one master control unit (e.g., ECU for a vehicle) 1 is connected to a plurality of slave control units 2, for instance 2(1) to 2(6), like a string via buses 3, specifically 3(+) and 3(−). A switch 4 is contained in each slave control unit 2. In case a succeeding slave control unit 2 becomes defective, the switch 4 works to disconnect the bus 3 from the defective slave control unit 2, so that the communication is continued by the normal slave control units 2 only.
FIG. 15 illustrates voltage waveforms with which the master control unit 1 or the slave control unit 2 drives the buses 3 when the data are to be transmitted relying upon the SbW communication. In the SbW, the master control unit 1, first, drives the buses 3 with a voltage level VLP in a power phase, to supply a drive power to the slave control units 2. A data phase that follows the power phase becomes a period in which the master control unit 1 or the slave control unit 2 transmits the data of one bit.
When the master control unit 1 transmits the data in the data phase, the buses 3 are driven to a voltage level of either VL0 or VL1. These voltages levels correspond to the data “0, 1”. In a period in which the slave control unit 2 transmits the data, the master control unit 1 drives the buses 3 to the voltage level VL0 in the data phase. The lengths of periods of the power phase and data phase have been specified to be equal.
In this case, if the slave control unit 2 does not drive the buses 3, the voltage level remains at VL0, meaning that the data “0” is transmitted. If the slave control unit 2 drives the buses 3 to VL1 of which the voltage level is lower than VL0, it means that the data “1” is transmitted. If the slave control unit 2 drives the buses 3 to VLS0 of which the voltage level is lower than VL1, it means that an interrupt is generated for the master control unit 1.
FIG. 16A illustrates a data communication between the master control unit 1 and the slave control units 2. The master control unit 1 drives the buses 3 to VLP and VL0 at a length twice as long as a bit period (power phase+data phase) at a communication rate at that moment to transmit SOF (start of frame), which is shown in FIG. 16B and represents the start of communication. With the SOF being transmitted onto the buses 3, the slave control units 2 recognize the start of communication.
Then, the master control unit 1 transmits two data bits, i.e., MSA and SEL. Thereafter, the slave control units 2 acquire a period for transmitting the data. That is, a Slot1_data and a subsequent CRC are data transmission periods of the master control unit 1(1), the CRC (cyclic redundancy check) being an error detection code attached concerning the Slot1_data. Similarly, hereinafter, a Slot2_data, a Slot3_data, - - - , a Slot_n_data are successively transmitted by the other slave control units 2.
The master control unit 1 is learning in advance the number of the slave control units 2 connected to the buses 3. When the data of a bit number corresponding to all slave control units 2 are transmitted, therefore, a series of communication ends. In the SbW, as described above, the slave control unit 2 supplies the power in the power phase while a data bit is being transmitted over two buses 3. By using only two buses 3, therefore, it is allowed to constitute a serial communication system capable of supplying the power.
If it is presumed that the above communication system is applied to an air bag device mounted on a vehicle, the plurality of slave control units 2 are corresponded to the acceleration sensors arranged at various portions of the vehicle. If any acceleration sensor detects the shock of an accident, the detection signal is transmitted from the slave control unit 2 to the master control unit 1. The master control unit 1 therefore sends an ignition instruction to an inflator in the air bag device that is not shown to generate a gas, so that the air bag inflates.
The SbW is a technology which has not been widely employed yet, and the applicant could not find any pertinent prior art document to be quoted.
The above communication system alternately repeats the power phase in which the master control unit 1 drives the buses 3 to supply the power and the data phase in which the slave control unit 2 can drive the buses 3. The SOF that represents the start of communication is, thereafter, output by the master control unit 1 onto the buses 3 followed, however, by no timing signal with which the slave control units 2 can maintain synchronism. That is, the Standards are not specifying how the synchronism be maintained by the slave control units 2 relative to the communication started by the master control unit 1 and, hence, the communication system basically is asynchronous.
Therefore, the slave control units 2 individually measure the periods of the power phase and the data phase to determine the arrival of their own periods for transmitting data, and drive the buses 3 in the data phase. Therefore, if the times measured by the slave control units 2 are deviated, it is probable that the end of data phase overlaps the head of the succeeding power phase. In this case, if the slave control unit 2 drives the buses 3 to either one of VL1 or VLS0 in the data phase and, then, if the master control unit 1 drives the buses 3 to VLP in the power phase, it becomes probable that the drive levels of the buses 3 interfere with each other in the above overlapped periods as shown in FIG. 17 and the communication may fail.